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What is a detergent anyway?

May 15, 2009

Detergents are all surface-active chemicals – they work in the space between the surface of a thing and the grease or particulate dirt to mechanically or sometimes chemically roll it up and remove it. For cosmetics we sometimes split “detergents” two general families – Soap based or Surfactant based and they are generally used to do the ‘de-greasing’ of your body – think shampoo, bar soap, bubble bath, foaming cleanser etc

So, what is the difference? How do they work?  and what do I need to do if I have sensitive skin?

The difference between Soap and Surfactants and How they Work.

In order to be technically correct we should always keep in mind that soaps ARE surfactants in as much as they work at the surface to remove dirt. However, for marketing purposes they are sometimes treated as different things!

Soap – One of the oldest types of surfactant,  is made by reacting an oil (pretty much anything goes – palm, olive, jojoba can be used) with an alkali solution (potassium hydroxide or sodium hydroxide are commonly used) – this is sometimes referred to as the lye. This reaction produces a salt of the oil and a bi-product called glycerin.  The part of the soap that does the cleaning,  is negatively charged (anionic) which is a good thing as your dirt is typically cationic (positive). You can see that quite an attraction could build up! The soap wets the surface by reducing surface tension (makes the water more spreadable), sticks to the dirt and starts to react with it  or saponify it. The dirt that has reacted gets washed away with the bubbles.

The above is the most basic way of making soap and can be tried at home with a little patience and some time. The  alkali part of the recipe is pretty harsh on the skin as it has a very high pH so care should be taken.  To ensure that all of the alkali has been used up in the reaction the pH of the bar needs to also be checked before use or you will find yourself with some skin burns!

There are lots of other ways of making soap. Different base oils will give different soap properties, the glycerine from the reaction can be left in or taken out (leaving it in makes the soap more moisturising and less drying to the skin). Finally, the alkali source can be changed – sodium hydroxide gives a harder soap than potassium hydroxide.  Further to that, we get into synthetic detergent and cleansing bars, which are then more like surfactants and less like the “soap” that has served us so well!  Loved  by many for its naturalness Castile soap does not contain any additives in the way of synthetic perfumes, preservatives or chelating agents (EDTA).   However, as it is still made by saponification which involves an alkali the soap still has a high pH and can be drying to some skins.

Surfactants – This is the name given to a wide range of “surface active” chemicals. They can be made up of entirely natural ingredients or can be synthetically produced. They can be mild and non-irritating or they can be extremely irritating to the skin. A lot depends on their ability to degrease a surface – your skin or hair!

Surfactants may be positively charged (cationic) – these are good for sticking to cleansed surfaces and are often used as the actives in conditioners, moisturizers, hair spritzers or polishes.  On a label, you could look out for things like Cetrimonium Chloride or quaternium 10.  Anionic surfactants are the workhorses of the surfactant family – these are negatively charged and can range from excellent degreasers that will irritate the skin to very mild, baby safe degreasers that gently remove dirt.  Think Sodium Lauryl Sulphate, Sodium Lauroyl Lactylate or Disodium laureth sulfosuccinate.  Finally there are the amphoteric surfactants, these have a dual charge and are often used as secondary surfactants to improve the foam quality or mildness of the primary surfactant. They also aid in the removal of dirt and can sometimes help to thicken the product. Think Cocoamidopropyl Betaine, Lauramine Oxide or Cocamidopropyl Hydroxysultaine.

So what is the difference between soap and surfactants? The chemical reaction taken to make them – saponification with alkali for soap and something else for other surfactants (may be ethoxylation or something else to turn the basic oil into something that has a surface active head, tail or both! All other properties can vary depending on what else you do to the formulation. However, soaps  like these traditional ones  work best at high pH whereas the other surfactants can work at a range of pH’s and most often are formulated to be closer to the skin’s own pH of around 5.5.

What should I do if I have sensitive skin?

There are hundreds of different detergents available on the market ranging from traditional bar soaps to cleansing bars to liquid soaps to “green” alternatives such as Castile soap (mentioned above).  If your skin is very sensitive then your dermatologist will probably advice “soap free” or “detergent free” cleaners and may prescribe something like an oatmeal bath product or an oil based bathing cream for you. When you have to use something a bit stronger you may be advised towards baby shampoo’s as these are formulated with surfactants that have been proven to be as mild and non-irritating as possible, often have little to no fragrance and are pH balanced to avoid giving your skin a shock.

If your skin is sensitive, traditional bar soap may be a bit too drying. Buying soap that contains glycerine will help to make it more moisturising but the shear fact that the soap has a high pH may be enough to send your skin into a frenzy. Castile soap, while touted as a green alternative also has a high pH.

If you have sensitive skin but want to buy cleansing products, I would stick to the following as a rule of thumb:

* pH balanced (to around 5.5)

* Fragrance and Colour Free.

* Marketed to babies or for sensitive skin on the label.

* Contain added moisturisers (glycerine is one but other oils may be present too).

If you want to make your own try either a cold cream ( a traditional cream recipe that acts like a cleanser and helps to remove dirt – see here. In this recipe, you could substitute the mineral oil for a vegetable-based oil of your choice.

You could also try wiping off dirt using an oil such as Macadamia nut or Jojoba – something that has a bit of grip on the skin (olive oil would be too slippery and messy). Alternatively, just try water, a bit of steam and a wet flannel – be gentle and moisturize after! Finally, you could try whipping out the old Bicarbonate of Soda!  When bicarb encounters dirt it starts to saponify it (turn it into a soap) which can then be washed away. The bicarb is a mild alkali so it could still irritate your skin but there is no doubt that it is a simple and effective degreaser in some situations – some people swear by it and even use it to rinse hair. BRILLIANT!

Therefore, to finish it is worth pointing out that this article only just touches the surface of surface chemistry – the science behind detergents!  You can clean your skin without detergents but most of the time they are used to help to remove grease in a convenient and pleasurable way.  Don’t forget the tool shed when it comes to degreasing your body and hair – try combs, sponges, flannels, pumice stones and de-greasing paper if you want to keep the carbon footprint in check. Check back for some more surfactant talk soon.

One Comment leave one →
  1. July 30, 2020 2:27 am

    No, actually, “detergent” is a much broader word, meaning “cleaner” as a noun, “cleaning” as an adjective. The word was in use before the chemistry of surface active agents was known, and substances can be detergents, or detergent, regardless of whether they have any surfactant content. Chromic acid made from dichromate and sulfuric acid solution used to “burn off” dirt from lab glassware is a detergent. Wet sand for scouring is a detergent. And so on.

    Not only that, but not all soaps are used for their surface activity. There are insoluble soaps used in greases and as anticaking and water-resistance agents, and then there are also short chain fatty acid salts that now technically fit the definition of “soap” but have no useful amount of surface activity.

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